If you shoot an ISO 100 slide at EI of 125, you are effectively underrexposing 1/3 stop. For slides, this usually result in enhanced colour saturation. For negatives, you won't get much of an effect as the lab will correct for this slight underexposure.
For negs, better to slightly overexpose. Or to make it simpler:
1. For slides, expose for highlights.
2. For negs, expose for shadows.